The invention relates to a mounting method for assembling a miser laser with fiber coupling.
A miser laser with fiber coupling miser may be required, for example, for optical telecommunications engineering with coherent transmission, since this requires an extremely narrowband and phase-stable signal light source. According to the prior art optical modulators with transverse single-mode and polarization-dependent optical fibers are used to modulate the light. Consequently, the light generated by the miser may need to be coupled into a polarization-maintaining single-mode fiber. Moreover, in the case of free-beam transmission between satellites, there may still be a need for optical amplification of the modulated signal in a single-mode fiber amplifier.
The method of functioning of a miser (MISER=Monolithic Isolated Single-mode End-pumped Ring resonator) has been discussed, for example, in the publications by Thomas J. Kane and Robert L. Byer xe2x80x9cMonolithic, unidirectional single-mode ND:YAG ring laserxe2x80x9d, Optics Letters, Vol. 10, No. 2, 65 (1985) and Thomas J. Kane, Emily A. P. Cheng, xe2x80x9cFast frequency tuning and phase locking of diode-pumped ND:YAG ring lasersxe2x80x9d, Optics Letters, Vol. 13, No. 11, 970 (1988). By contrast with a laser having a rod-shaped laser crystal and two end mirrors, in a ring laser the light is guided on an annular light path via a plurality of deflecting mirrors. In this case, both senses of circulation are possible for the light.
In a miser laser, the light is guided inside a monolithic laser crystal on a non-planar annularly closed light path. The light guidance is performed by a plurality of total reflections at the crystal faces and by partial reflection at a light exit and entry surface, which has a high reflectivity for the laser light. The pumping light is launched longitudinally, that is to say in the beam direction of the laser light by the same partially reflecting surface from which the laser light exits. Consequently, this surface may need to be dichroically coated and have a low reflectivity for the pumping light and a high reflectivity for the laser light.
By comparison with arrangements with separately assembled mirrors, the monolithic integration of all the reflecting mirrors as bounding faces of a single crystal produces a substantially more robust beam guidance. A miser laser is distinguished by a laser exit beam which is a single-mode beam in longitudinal and transverse terms. In order to achieve a stable longitudinal single-mode characteristic, standing waves may need to be avoided in the amplifying range of the laser, since, because of what is termed a spatial-whole-burning effect in the region of the antinodes of a standing wave, inversion there may preferably be reduced. In the middle region of the resonator, however, the next higher or lower longitudinal mode of a laser has its nodes at these points, and so their excitation may be preferred, which may lead to uncontrolled jumping between adjacent longitudinal modes, and thus to frequency instability.
In order to avoid standing waves in the amplifying range of the laser, it may be necessary to suppress one sense of circulation in the case of an annular light path. This is done in the case of a miser by superimposing a reversible and an irreversible rotation of the plane of polarization of the light. The reversible rotation of the plane of polarization is performed in the case of the miser by virtue of the fact that the light path is not guided in a plane. In this case, the light tracks at least three reflecting surfaces whose normal directions do not lie in a plane. This leads to a rotation of the direction of polarization of the light with reference to the light propagation direction which has an opposite direction for the two annularly guided light paths in accordance with their sense of circulation.
A material exhibiting a Faraday effect such as, a Nd-doped YAG crystal, is used for the miser laser in order to generate the superimposed irreversible rotation of the polarization direction. For this purpose, a magnetic field is applied parallel to the longitudinal axis of the YAG crystal.
The effect of superimposition of the two effects in relation to reversible and irreversible rotation of polarization is that the two rotational effects are amplified for one direction of circulation of the light, and weakened for the other direction of circulation. Given a suitable strength of the magnetic field, when striking the light exit surface, the light of one direction of circulation therefore strikes virtually at right angles to and, in the other direction of circulation, virtually parallel to the plane of incidence. This results in a different reflection and transmission response for the two directions of circulation and, as a consequence thereof, a different degree of damping. Consequently, only the light wave with the direction of circulation in the direction with the minor damping obtains laser operation. Standing waves are thereby avoided and a longitudinal single-mode operation is rendered possible.
The transverse single-mode characteristic is achieved by means of a longitudinal pumping light excitation via the light entry surface, the cross section of the pumping light beam being limited such that possibly only the volume of the transverse fundamental mode is excited. The tuning of the frequency of the laser light can be performed by temperature control of the laser crystal with the aid of a relatively large time constant and a relatively large frequency deviation and, additionally, by means of mechanical bracing via an adhesively attached piezoelectric element with a smaller time constant in the millisecond range and a smaller frequency deviation.
A mounting and assembling concept for a miser is proposed in U.S. Pat. No. 4,749,842 dated Jun. 7, 1988, the light source for the pumping light being integrated in the same housing as the miser crystal. The laser beam from the miser exits as a collimated beam from a window in the housing wall.
Stable launching of the light into a polarization-maintaining single-mode fiber may be required for the above-described application of a coherent optical message transmission. The difficulty may arise here that both the beam path for the pumping light and the beam path for launching the laser light into the polarization-maintaining single-mode fiber may need to be aligned with high accuracy in relation to the laser crystal.
A further difficulty may arise because retro-reflections of the output beam back into the laser crystal may need to be avoided in order not to endanger the frequency stability of the miser. In the previous solution, the laser crystal is adjusted in a plurality of degrees of freedom in relation to the incident pumping light beam, and fixed by soldering. In this adjustment, the ideal pumping light launching site may need to be struck on the light entry and light exit surface on the laser end face within a small region, with a diameter of a few tens of micrometers, at the correct angle since it is only starting from this launching site that an inherently closed light path is possible and a laser beam can be excited. According to the prior art cited above, the laser crystal may be adjusted in relation to the focused pumping light beam in three translation coordinates and two angular coordinates until a laser beam of optimal power emerges from the crystal. The direction and position of the exiting laser beam depends on the position of the laser crystal after the adjustment, and is therefore undetermined. As a consequence of tolerances in the dimensions and cut angles of the crystal, this can result in a substantial uncertainty in the direction and position of the laser beam.
In order to focus the laser beam onto a polarization-maintaining single-mode fiber for applications to coherent transmission technology, there may be a need for a focusing lens which may need to be positioned in the beam path. This lens cannot be fitted directly at the exit window of the laser crystal since optical delay plates may also be required for setting the state of polarization between the laser crystal and focusing lens. In an adjusting and assembling concept, according to the prior art, coupling a single-mode fiber would be very problematic, since the adjusting operation of the laser output depends on the adjusting operation at the pumping light input and the uncertainty, caused thereby, in the position of the laser beam.
The present invention proposes an assembling and adjusting concept which avoids the above-described disadvantages and permits the pumping light input and the laser output to be coupled stably and with low retro-reflection into a polarization-maintaining single-mode fiber, the adjusting processes at the pumping light input and at the laser output being independent of one another.